Study of AToM "chip9 / LG" problem


The "Chip 9" or "Log Gain (LG)" problem of the AtoM chip was first observed in Santa Barbara and then also in Pisa/Milano. Briefly, the symptoms of the problem are:

It has been hypothesized that the problem may be due to a problem in the internal charge injection circuitry. This can be verified by examining the behaviour under external charge injection. We understand that members of the Pavia group will be performing measurements by injecting charge through externally-bonded capacitors. Here we report on measurements performed at UCSB on a bonded detector using a focussed infrared LED source.

The measurements were performed on DFA D01B-6, bonded to HDI H1-18. We performed a series of threshold scans both with internal charge injection and using an LED pulser. The amount of charge deposited via the LED was controlled by varying the length of the pulse used to fire the LED. This length varied typically between 15 and 55 nsec; the risetime of the LED is of order 5 nsec. The size of the LED spot on the detector was approximately 600 micron (FWHM). All measurements reported here used a 40 MHZ clock and a shaping time of 100 nsec.

In Figures 1 and 2 we show threshold turn-on curves from internal charge injection for a channel on the n-side and a channel on the p-side.






Figure 1: Threshold turn-on curves under internal charge injection for a channel on the n-side (chip 3 chan 41) and a channel on the p-side (chip 9 chan 24). These correspond to a charge injection of 2 CAL DAC counts and firing 16 channels simultaneously.






Figure 2: Same as Figure 1, but firing 8 channels simultaneously.




The fact that chip 9 on the p-side is affected by the Chip 9/LG problem in the run taken in Figure 2 is not immediately obvious. In general, it is a little harder to spot the problem simply by inspecting the threshold curves after bonding than it is before bonding (due to the slow turn on in the curves after bonding). However, the chisquared distributions for the error function fits are a good way to spot the Chip 9/LG problem, see Figures 3 and 4.








Figure 3: Distribution of chisquared for the threshold turn on curves of chip 3 (n-side) and chip 9 (p-side). These are for charge injection of 2 CAL DAC counts and firing 16 channels simultaneously. No (or little) evidence of Chip 9/LG problem.






Figure 4: Same as Figure 3, but for firing 8 channels simultaneously. The Chip 9/LG problem is apparent.




We now show in Figure 5 the chisquared distribution for threshold turn-on curves obtained when externally injecting charge in the front end by shining the infrared LED on the detector. Note that the statistics are small because only a few channels per side were illuminated by the LED. However, a wide range of charges was explored, see the caption to Figure 5.








Figure 5: Distribution of chisquared for the threshold turn-on curves of chip 3 (n-side) and chip 9 (p-side). These are for charge injection using the infrared LED. The points here have been accumulated in a number of different runs, where the LED intensity was varied by varying the length of the LED pulse. The charge collected per strip ranged approximately between 1 and 3 fC.




There is no difference in the chi-squared distributions for chip 9 and chip 3 in Figure 5. Therefore, at least for the one chip that we studied, we conclude that there is no evidence of Chip 9/LG problems when examining threshold turn-on curves under external charge injection.

The other very serious manifestation of the chip 9/LG problem is a (possibly) reduced gain. We can measure the relative gain of all chips because we are able to control the amount of deposited charge by varying the length of the LED pulse. We appear to be operating in a very linear region, see Figure 6. We are now analyzing these data to determine whether the gain of chip 9 is anomalously low. This work is not finished, but preliminary results are very encouraging. Stay tuned.








Figure 6: Distribution of 50% threshold turn on point vs the length of the LED pulse for two channels on the p-side. Note that thresholds decrease bottom to top, i.e. THR DAC = 0 is the highest threshold, THR DAC = 63 is the lowest threshold. Channel 25 is at or near the maximum of the light intensity, channel 29 is further away, hence the difference in slopes.

New!(12 August 1998)
Information on chip 9 / LG gains is now available


Claudio Campagnari Page Last Updated: Aug 12, 1998
Bryan Dahmes
Stephen Levy
Owen Long